Determining stowability based on item-size categories

ABSTRACT

Methods, systems and apparatus for determining stowability based on item-size categories are described. Embodiments may select a storage module for a stow operation based on accessing stowability information. Stowability information, in various embodiments may include stowability information for each storage module in a materials handling system. Stowability information may indicate a capacity to hold additional items in a storage module. The capacity may be determined according an item-size category descriptor, describing the quantity of items in each category of a storage module, and a storage module capacity module characterizing item descriptors of fully-stowed storage modules. In some embodiments the storage module capacity model is a function fitted to data points representing the values of item-size descriptors of fully-stowed storage modules. Embodiments may update the stowability information for a storage module in response to the removal of an item from the storage module.

This application is a divisional of U.S. patent application Ser. No.13/528,214, filed Jun. 20, 2012, now U.S. Pat. No. 8,825,197, which ishereby incorporated by reference in its entirety.

BACKGROUND

Stocking various items received at one or more distribution centers,fulfillment centers, cross-docking facilities, materials handlingfacilities or warehouses (which may collectively be referred to asmaterials handling facilities) is a time consuming process in a typicaldistribution system. Generally, the stocking process, or stowing processas it sometimes referred to, transports items from a receiving area andplaces the received items into an inventory storage area in a materialshandling facility. Items are placed in storage units with enough storagecapacity to hold the additional items. Determining which storage unithas capacity to hold additional items is conventionally determinedmanually. A human stocker must visually inspect the storage units tolocate a particular storage unit with sufficient capacity for the itemsready to be stocked. This process proves time intensive when stockersmust search over large inventory areas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example control system which may direct stowoperations according to storage capacity, according to some embodiments.

FIG. 2 illustrates a logical representation or view of the operation ofa materials handling facility, according to some embodiments.

FIG. 3 illustrates a high-level flowchart of a method to select astorage module based on stowability information, according to someembodiments.

FIG. 4 illustrates a high-level flowchart of a method to generate astorage module capacity module based on item-size category descriptors,according to some embodiments.

FIG. 5 illustrates an example of a storage module capacity model as afunction fitted to describe item-size category descriptors, according tosome embodiments.

FIG. 6 illustrates a high-level flowchart of a method to update theitem-size category descriptor of a storage module in stowabilityinformation, according to some embodiments.

FIG. 7 illustrates an example of a physical layout of the inventory areaof a materials handling facility in which mobile robotic devices maytransport storage modules to stow stations, according to someembodiments.

FIGS. 8A and 8B illustrate views of an example mobile robotic devicewhich may transport storage modules to stow stations, according to someembodiments.

FIGS. 9A and 9B illustrate views of an example storage module, accordingto some embodiments.

FIG. 10 illustrates an example of a mobile robotic device transporting astorage module, according to some embodiments.

FIG. 11 illustrates an example of a physical layout of the inventoryarea of a materials handling facility in which items may be transportedto an inventory storage area, according to some embodiments.

FIG. 12 illustrates an example system, according to some embodiments.

While embodiments are described herein by way of example for severalembodiments and illustrative drawings, those skilled in the art willrecognize that embodiments are not limited to the embodiments ordrawings described. It should be understood, that the drawings anddetailed description thereto are not intended to limit embodiments tothe particular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope as defined by the appended claims. The headings usedherein are for organizational purposes only and are not meant to be usedto limit the scope of the description or the claims. As used throughoutthis application, the word “may” is used in a permissive sense (i.e.,meaning having the potential to), rather than the mandatory sense (i.e.,meaning must). Similarly, the words “include,” “including,” and“includes” mean including, but not limited to.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following detailed description, numerous specific details are setforth to provide a thorough understanding of claimed subject matter.However, it will be understood by those skilled in the art that claimedsubject matter may be practiced without these specific details. In otherinstances, methods, apparatus, or systems that would be known by one ofordinary skill have not been described in detail so as not to obscureclaimed subject matter.

It will also be understood that, although the terms first, second, etc.may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first contact could be termed asecond contact, and, similarly, a second contact could be termed a firstcontact, without departing from the scope of the present invention. Thefirst contact and the second contact are both contacts, but they are notthe same contact.

The terminology used in the description of the invention herein is forthe purpose of describing particular embodiments only and is notintended to be limiting of the invention. As used in the description ofthe invention and the appended claims, the singular forms “a”, “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will also be understood that theterm “and/or” as used herein refers to and encompasses any and allpossible combinations of one or more of the associated listed items. Itwill be further understood that the terms “includes,” “including,”“comprises,” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof.

As used herein, the term “if” may be construed to mean “when” or “upon”or “in response to determining” or “in response to detecting,” dependingon the context. Similarly, the phrase “if it is determined” or “if [astated condition or event] is detected” may be construed to mean “upondetermining” or “in response to determining” or “upon detecting [thestated condition or event]” or “in response to detecting [the statedcondition or event],” depending on the context.

Various embodiments of methods, systems, and apparatus for determiningstowability based on item-size categories in materials handling facilityare described. A materials handling facility, such as one or moredistribution centers, fulfillment centers, cross-docking facilities,materials handling facilities or warehouses, may receive items forstorage in an inventory area. The process of stocking items, oftentimesand hereinafter referred to as stowing items, into the inventory areamay involve transporting received items and stowing them in one of manystorage modules located in the inventory area. Storage modules mayalready hold varying amounts of items. Additionally, storage modules mayhave different sizes or configurations which affect the storage capacityof a particular storage module to hold additional items. Selecting whichstorage module to stow items into may be accomplished by comparing thestowability, the capacity of a storage module to hold additional items,of the storage modules to identify storage modules with the moststowability.

In some embodiments, a control system may be implemented in a materialshandling facility. When items received at the materials handlingfacility are ready to be stowed, the control system may direct a stowoperation, performed by a human or robotic device, to stow the itemsinto a selected storage module. The control system may select from amongthe many storage modules in a materials handling facility to stow itemsinto by comparing the stowability of the storage modules and selecting astorage module with the most stowability. The control system may haveaccess to the stowability information of the storage modules.

Stowability information accessible by a control system may includestowability information for each storage module in the materialshandling facility. Stowability information for a storage module mayinclude a capacity of the storage module to hold additional items. Thecapacity to hold additional items may be based on a storage modulecapacity model. The storage module capacity model may describe variousconfigurations of items in fully-stowed storage modules. To determinethe capacity to hold additional items for a storage module, variousembodiments may apply the storage module capacity model to the itemsalready held in the storage module, which are categorized into differentitem-size categories. Items may be identified as belonging to anitem-size category according to the size information for the particularitem.

In various embodiments, a control system may track the capacity ofstorage modules to hold additional items, updating the stowabilityinformation as items are picked, or removed, from the storage modules.The control system may also update the stowability information for astorage module as stow operations stowing items into the storage moduleare completed.

In some embodiments, a control system may direct mobile robotic devices(referred to herein as bots) to transport storage modules from theinventory area to stow stations. At the stow stations, human orautomated stowers may perform stow operations, stowing received itemsinto storage modules. When the storage module is fully stowed, a controlsystem may direct a bot to return the storage module to the inventoryarea of the materials handling facility.

Embodiments of the method for determining stowability based on item-sizecategories may, for example, be implemented in a materials handlingfacility, such as depicted in FIG. 7, including a stow process thatemploys stow stations. Bots may transport storage modules to stowstations where stowers may stow items into the storage modules. However,embodiments of the method for determining stowability based on item-sizecategories may be implemented in other materials handling facilitiesthat employ other stow process, such as stow processes that direct humanor robotic stowers to the storage modules in an inventory area, such asillustrated in FIG. 10, to perform stow operations. More generally,embodiments of the method for determining stowability based on item-sizecategories may be implemented in any materials handling facility thatstows received items into an inventory area.

Example Control System

Various embodiments may implement a method for determining stowabilitybased on item-size categories in a control system in a materialshandling facility. FIG. 1 illustrates an example control system whichmay implement a method for determining stowability based on item-sizecategories to direct stow operations, such as selecting a storage modulewith the most stowability, according to some embodiments. The controlsystem depicted in FIG. 1 may be implemented on one system or computingdevice, such as system 900 described below with respect to FIG. 12, orimplemented across several computing systems or devices. In someembodiments, the control system in FIG. 1 may be part of a largercontrol system directing the various operations of a materials handlingfacility discussed below with regard to FIG. 2.

A control system, such as control system 100, may obtain informationfrom various other systems or components in a materials handlingfacility. Communications between the control system and other systemsmay occur over a variety of standard or customized communicationtechnologies, such as wired or wireless networks. For instance, controlsystem 100 may receive item-size descriptors for fully stowed modules110 from some measurement system or database. Embodiments of a controlsystem 100 may obtain this information to direct stow operations.

In some embodiments, a control system may select a storage module fromamong the storage modules in a materials handling facility to stow itemsinto based on stowability information for the storage modules. Storagemodule selector 160 may be configured to access stowability information150. Stowability information 150 may be a database or other variety ofdata storage which stores stowability information for the storagemodules in a materials handling facility. As discussed above thestowability information for storage modules may be the capacity ofstorage modules to hold additional items. Stowability information 150may located external to control system 100 or located internally (notdepicted).

The capacity of storage modules to hold additional items as indicated instowability information 150, may be determined, tracked, or updated by astowability information state manager 140 implementing various methodsdiscussed below with regard to FIGS. 3 and 6. To determine thestowability for storage modules, stowability information state manager150 may be configured to apply a storage module capacity model 130 tothe quantity of items held in a storage module, categorized intoitem-size categories. The quantity of categorized items held in astorage module may be described using an item-size category descriptor.

A storage module capacity model 130 may be generated by a controlsystem's storage module capacity model generator 120. Storage modulecapacity model generator 120 may be configured to generate one or morestorage module capacity models for control system 100. The storagemodule capacity model generator 120 may be configured to obtainitem-size descriptors for fully-stowed modules to generate storagemodule capacity models, such as storage module capacity model 130,implementing the various methods described below with regard to FIGS. 4and 5. In some embodiments, multiple storage module types may be usedsuch that storage module capacity model generator 120 may generatestorage module capacity models for each storage module type.

As items are picked from storage modules, described below at 260 in FIG.2, stowability information 150 may be updated by the stowabilityinformation state manager 140 to reflect the removal of the items fromthe inventory area. Particular methods and embodiments for updating thestowability information 130 are discussed below with regard to FIGS. 3and 6. The stowability information state manager 140 may be configuredto obtain item pick and stow information for storage modules 180. Forinstance, a system or device external to the control system 100 may sendpick information over wired or wireless networks to the control system.For example, in some embodiments as items are picked from storagemodules or stowed in storage modules, an inventory management system mayindicate over a wired network to the control system 100 that certainitems have been picked or stowed or are scheduled to be picked or stowedin a particular storage module. A robotic or human picker or stower mayoperate some access device to indicate in the inventory managementsystem picked or stowed items. In another example, a control system 100may be configured to receive from a portable inventory reporting devicereports of picked or stowed items. A portable inventory reporting devicemay be a mobile or portable, computing or electronic device capable ofreporting picked or stowed items directly to a control system 100 or toan inventory management system which then reports the picked or stoweditems to the control system 100. Such a device may be a handheldelectronic device that scans item barcodes or radio frequencyidentification (RFID) tags to indicate over a wireless network to thecontrol system 100 that the scanned item has been picked from aparticular storage module.

The updating and tracking provided by the stowability information statemanger 140 to stowability information 150 may allow storage moduleselector 160 to select storage modules with the most stowability for astow operation. Storage module selector 150 may be configured toimplement the various methods and embodiments for selecting a storagemodule discussed below with regard to FIG. 3. Control system 100 maydirect a stow operation for stowing items on the select storage module170. Various communication methods and devices may be targeted for thedirected stow operation. For example, in some embodiments a portableinventory reporting device, such as discussed above, may include adisplay screen that displays stow directions received over a wirelessnetwork from the control system for received items. The portableinventory reporting device may display stow directions for one or moreitems to be stowed in a selected storage module. In another example,some embodiments may employ one or more stow stations where itemsreceived at the materials handling facility may be stowed into a storagemodule. In at least some embodiments, a control system 100 may direct abot over a wireless network to transport the selected storage module 170from an inventory area to a stow station. Upon completion of the stowoperation, the control system 100 may direct a bot to transport thestorage module 170 back to the inventory area. A control system 100 maydirect a stow operation for stowing items in a selected storage moduleto various other devices, such as display device viewable by humanstowers.

Overview of Materials Handling Facility Operation

In various embodiments a control system may be implemented in amaterials handling facility. FIG. 2 illustrates a logical representationor view of the operation of a materials handling facility in whichembodiments of methods, systems, and apparatus for determiningstowability based on item-size categories may be implemented. Forexample, this Figure may illustrate an order fulfillment center of aproduct distributor. Multiple customers 240 may submit orders 250 to theproduct distributor, where each order 250 specifies one or more itemsfrom inventory 222 to be shipped to the customer that submitted theorder. To fulfill the customers' orders 250, the items specified in theorders 250 may be retrieved, or picked 260, from inventory 222 (whichmay also be referred to as stock storage) in the materials handlingfacility. The picked items may be conveyed, by the conveyance mechanismto one or more stations in the materials handling facility fordownstream processing, for example sorting 270 into their respectiveorders, packing 280, and finally shipping 290 to the customers 240.

A materials handling facility may also include a receiving 220 operationfor receiving inventory items 210 from one or more sources (e.g.,vendors) and for stowing the received items according to a stowingprocess 230 into item storage (inventory 222). The receiving 220operation may also receive and process returned purchased or renteditems or orders from customers. At least some of these returned itemsare typically returned to inventory 222.

In at least some embodiments, rather than stowing directly to inventory222 as shown in FIG. 2, received inventory may be delivered to one ormore stowing stations (not shown). Under direction of a control system,such as described above with regard to FIG. 1, storage modules may beautomatically moved from inventory 222 to particular stowing stations,for example using the robotic devices. At the stowing station(s), thereceived inventory may be manually or automatically placed in locationson the storage modules under direction of the control system. Thestorage modules are then returned from the stowing station(s) toinventory 222, for example using the robotic devices. In someembodiments, induction and stowing may be combined at one station sothat individual items for orders can be inducted into the conveyancemechanism from storage modules for delivery to one or more downstreamprocessing stations (e.g., sorting stations) and received items can bestowed to the same storage modules prior to the storage modules beingreturned to inventory 222; storage modules may also be moved frominventory 222 to the induction/stowing station(s) specifically forstowing.

The various operations of a materials handling facility may be locatedin one building, or alternatively may be spread or subdivided across twoor more buildings. In addition, a materials handling facility mayinclude one or multiple levels or floors. For example, a materialshandling facility that includes inventory 222 may include one, two, ormore levels; in multi-level facilities, inventory 222 may be spreadacross two or more levels. The total floor space of a materials handlingfacility may be anywhere from tens of thousands of square feet to morethan a million square feet, although embodiments may be implemented insmaller facilities.

Workflow for Selecting a Storage Module

Embodiments of determining stowability based on item-size categories maybe implemented in a control system, such as described above with regardto FIG. 1. A control system may select a storage module based onstowability information and direct performance of a stow operation forstowing items received at the materials handling facility into theselected storage module. FIG. 3 illustrates a high-level flowchart of amethod to select a storage module based on stowability information,according to some embodiments.

Embodiments may receive items 310 at a materials handling facility asdiscussed above with regard to receiving 220 in FIG. 2. Items may bereceived from various sources including manufacturers, otherdistributers, or from customers returning items. Received items may beplaced into receiving and transported from receiving to the inventoryarea. In some embodiments, the received items may be transported tovarious stow stations in a stow area.

A control system, such as control system 100 in FIG. 1, may selectstorage module out the multiple storage modules in the materialshandling facility based on stowability information 320. A control systemmay be configured to access stowability information for the storagemodules. The stowability information may be in a separate database orother data storage device, such as stowability information 150 describedabove in FIG. 1, or as part of a larger database or system, such as aninventory management system. The stowability information for eachstorage module may indicate a capacity of the storage module to holdadditional items. The capacity indicated may be a numeric value orpercentage. The capacity of the storage module to hold additional itemsmay be determined according to a storage module capacity model, such asa storage module capacity model 130 generated by storage capacity modelgenerator 120 discussed above with regard to FIG. 1, applied to anitem-size category descriptor for items currently held in the storagemodule. An item-size category descriptor may include a quantity valuefor each different item-size category. Each quantity value may indicatea total quantity of items stored in the storage module categorized inone of many different item-size categories.

For example, an item-size category descriptor for a storage module mayinclude 10 small item-size category items, 6 medium item-size categoryitems, and 2 large item-size category items. The capacity to holdadditional items for the storage unit may be determined by applying astorage module capacity model, such as a function that describes fullcapacity storage modules described below with regard to FIGS. 4 and 5,to this information to determine the capacity of the storage module. Iffor example, 10, 6, and 2 were plotted in a space of three dimensions,with each dimension corresponding to small, medium, and large, thedistance from the point at (10, 6, 2) to the model function mayrepresent the space left for items that may be additionally stored inthe storage module before it is fully-stowed. Thus, this distance may beused to indicate the capacity of the storage module to hold additionalitems. Various methods and embodiments for generating a storage modulecapacity module and determining the capacity of a storage module arediscussed below with regard to FIGS. 4 and 5.

A control system may select a storage module for a stow operation basedon the accessed stowability information. In some embodiments, thecontrol system may select the storage module having the most capacity tostore additional items. For example, if the capacities for storagemodules are indicated as numbers or percentages, a control system mayselect the storage module with the greatest indicated number orpercentage. Embodiments may also base selection decisions on otherinformation in addition to the accessed stowability information. Forexample, a control system may select a storage module with a capacity tohold additional items above a certain threshold and within a certainradius from a particular stow station.

Various embodiments may direct performance of a stow operation 330stowing items received at the materials handling facility into theselected storage module. For example, a control system may display amessage on a display device that a stower might read, or print outdirections to a particular storage module where the stower, human orrobotic, may stow items. Some embodiments may direct a bot to transportthe selected storage unit to a stow station. At the stow station,received items may be stowed into the selected storage unit. Uponcompletion of the stow operation, some embodiments may direct a bot totransport the storage module back to the inventory area.

Some embodiments may track the capacity of the storage modules andupdate the stowability information for the storage module 340 uponcompletion of a stow operation stowing items into a storage module. Insome embodiments the stowability information may be updated in responseto stowing a single item into a storage module. In other embodiments,the stowability information may be updated upon an indication that thestorage module currently being stowed is full. Updating the stowabilityinformation may include updating the quantity values of the item-sizecategory descriptor to reflect the additionally stored items. Theitem-size category for items stowed in a storage module may bedetermined, for example by accessing item information in an itemdatabase, and the updating the quantity value for the item. For example,if a storage module had prior to a stow operation 1 small, 2 medium and0 large items, and 4 medium items and 3 large items were determined tobe stowed in the storage module after which the storage module wasindicated full, the quantity values in the item-size category descriptorfor the storage module would be 1 small, 6 medium, and 3 large items.Some embodiments may update the indicated capacity to hold additionalitems in the stowability information according to the various methodsdiscussed below with regard to FIGS. 4 and 5. In some embodiments anitem database tracks the location of stowed items, and the itemstowability information communicates with the item database to obtainthe item-size category descriptor for a storage module and indicates theupdated capacity according to the information obtained from the itemdatabase.

Workflow for Generating a Storage Module Capacity Model

A control system, such as discussed above with regard to FIG. 1 mayimplement various components to determine stowability based on item-sizecategories. In some embodiments, control system including storage modulecapacity model generator 120 may implement various methods to generatemultiple storage module capacity models. FIG. 4 illustrates a high-levelflowchart of a method to generate a storage module capacity module basedon item-size category descriptors, according to some embodiments.

Various embodiments may identify an item-size category for each item 410in a materials handling facility. These items may be still in receiving,such as discussed above with respect to receiving 220 in FIG. 2, or maybe currently stored in the inventory area in various storage modules.Multiple item-size categories may exist and use descriptive labels, suchas small, medium, and large. However, non-descriptive labels, such ascategory 1, category 2, and category 3 may be used. The number ofitem-size categories may not be limited, and therefore the number ofitem-size categories may be optimized according to the sizes or types ofitems received at the materials handling facility. For example, if amaterials handling facility specialized in distributing bed mattresses,the number of item-size categories may be 4, for twin, full, queen andking size mattresses. If in another example, a materials handlingfacility carried a wide range of items, the item-size categories may beless optimized and more descriptive, such as small, medium, large, andextra-large. The item-size category for each item may be determinedaccording to the size information for the item. Each item-size categorymay be defined as a range. Each range may be defined as a range for asize dimension, such as small is described by a range of item widthbetween 1 and 6 inches, or defined as a range in terms of space, such assmall items are from 1 to 10 cubic inches. In various embodiments theboundaries between item-size categories may be adjacent to one another,such as small 1 to 5, medium 6 to 10, and larger 11 to 20, or leavegaps, such as small is 1 to 4, medium 6 to 10, and large 12 to 20. Insome embodiments item size information may be inconsistent and the sametype of item may be categorized into a different item-size category. Forexample, the size information for a bag may be measured without anypacking for the bag. If then another bag of the same type of bag ismeasured in a shipping box, then the same bag may be in two differentitem-size categories. Various methods and techniques for sorting itemsinto item-size categories may be implemented which are well-known tothose of ordinary skill in the art in addition to the examples above.Therefore, none of the above examples are intended to be limiting.

Size information for items may be obtained from various sources. Somesize information for items may be obtained from the manufacturer ordistributer who shipped the item to the materials handling facility.Size information may be obtained from an item database or item, orinventory, management system, storing item size information according toeach uniquely identified item. For example, when an item is received ata materials handling facility the item size information may be obtainedfrom an item database via a globally unique identifier for the item inthe database. Size information may also be obtained by measuring itemsreceived at a materials handling facility, such as by measuring thedimensions of an item, e.g., using an optical item dimension scanner(e.g., a CUBISCAN system), or other measurement tool or technique.

Embodiments may then determine an item-size category descriptor forfully-stowed storage modules 420 in a materials handling facility. Asdiscussed above the item an item-size category descriptor may berepresented as several values corresponding to the quantity of items inan item-size category stored in a storage module. Fully-stowed storagemodules may be indicated to a control system by an external database,such as an inventory management system, or be indicated as fully-stowedin stowability information, such as stowability information 150 inFIG. 1. Fully-stowed storage modules may also be identified as modulesthat have completed a stow operation and are returned to the inventoryarea. For fully-stowed storage modules in a materials handling facility,a control system may access stowability information or an inventorymanagement system to determine the quantity value of items in eachitem-size category in the full capacity storage modules. The item-sizecategory descriptor representing these values may be a data structure orformat that represents both quantities and types of item-sizecategories. In some embodiments, the item-size category descriptor isstored as a data point in n-dimensional space, where each dimensioncorresponds to an item-size category. For example, if item-sizecategories are small, medium, large, and extra-large, the item-sizecategory descriptor would be a data point in 4-dimensional space for agiven storage module with 1 small, 1 medium, 1 large, and 1 extra-largeitem represented as (1, 1, 1, 1).

Embodiments may generate a storage module capacity model 430 based onthe item-size category descriptors for the fully-stowed storage modules.A storage module capacity model may characterize item-size categorydescriptors for storage modules at fully-stowed with items. In someembodiments, each item-size category descriptor for the fully-stowedstorage modules may be mapped as data points in an n-dimensional space.Each dimension of the n-dimensional space may correspond to a differentitem-size category. Each quantity value of the item-size categorydescriptor may map to a different dimension on the n-dimensional space.A function may be fitted to the data points in the n-dimensional spaceto describe the item-size category descriptors for the full capacitystorage modules. In some embodiments, several storage module types mayexist for storage modules. Storage module capacity models may begenerated according to the discussed methods for each storage moduletype.

FIG. 5 illustrates an example of a storage module capacity modelrepresented as a function fitted to describe item-size categorydescriptors, according to some embodiments. In the Figure, then-dimensional space 500 includes two dimensions, a small size-categorydimension 502 corresponding to a small item-size category and a largesize-category dimension 504 corresponding to a large item-size category.The item-size category descriptors 520 are mapped according to thenumber of small and large item-size category items in each fully-stowedstorage module. Using the data points, various well-known techniques,such as linear or non-linear regression techniques, may be used to fit afunction to the data points, such as the storage module capacity modelfunction 510. Note that the drawing in FIG. 5 appears to display a linein the space for the storage module capacity model function 510.However, as the number of dimensions may be up to n values, a functionmodel may be a line, plane, or multi-dimensional shape, and as such themodel depicted at 510 in FIG. 5 is not intended to be limiting.

In some embodiments, to determine the capacity of the storage module tohold additional items the item-size category descriptors for itemscurrently held in the storage module may be represented as values. Eachvalue corresponds to a different item-size category, and each valueindicates a total quantity of items stored in the storage modulecategorized in one of the item-size categories. For example, if astorage module has an item descriptor that represents the 3 small, 4medium, and 5 large items, the values may be 3, 4, and 5. Embodimentsmay then map the values to an n-dimensional space with each dimension ofthe n-dimensional space corresponding to an item-size category. Forexample, in FIG. 5 the item size category descriptors 520 may be thevalues of the item-size category mapped to the dimensions small, medium,and large. Embodiments may then calculate a distance from the locationof the values in the n-dimensional space to the function for the storagemodule capacity model. FIG. 5 illustrates the calculated distance 530between a location of a value in the n-dimensional space 500 and thestorage module capacity model function 510. Note that the drawing inFIG. 5 appears to display a line in the space for the storage modulecapacity model function 510. However, as the number of dimensions may beup to n values, a function model may be a line, plane, or shape, and assuch the model depicted at 510 in FIG. 5 is not intended to be limiting.Calculating the distance between the value and the function may beimplemented using various well-known techniques, such as calculating thedifference between the values represented as a vector and the function.Numerous other techniques may be employed to determine the distance andthe above example is not intended to be limiting.

Workflow of Updating Stowability Information

As various items are stowed or picked from storage modules in aninventory area in a materials handling facility, various embodiments ofa control system, such as control system 100 described with regard toFIG. 1, may track the capacity of the storage modules in a materialshandling facility to hold additional items. FIG. 6 illustrates ahigh-level flowchart of a method to update the item-size categorydescriptor of a storage module in stowability information, according tosome embodiments.

Various embodiments may receive an indication of an item being removedfrom a storage module 610, such as the item pick information 180described above with regard to FIG. 1. As discussed above, varioussystems and devices may communicate with a control system, such as overwired or wireless networks, to indicate an item's removal from a storagemodule. For example, a portable inventory reporting device, such asportable inventory reporting device discussed above with regard to FIG.1 may send a wireless indication to a control system of an item beingpicked from a storage module. Likewise, an inventory management system,such as an inventory management system discussed above in FIG. 1, mayalso communicate an indication of an item's removal to a control system.

In response to receiving an indication of an item being removed fromstorage module, a control system, such as using a stowabilityinformation state manager 140 described above in FIG. 1, may update theitem-size category descriptor in the stowability information for thestorage module to remove the item from the quantity value for theitem-size category of the removed item 620. Embodiments may determinethe item-size category for the removed item. This determination may bemade using item size information, such as discussed above at 410 in FIG.4, to identify an item-size category for the removed item. Once theitem-size category of the removed item is determined, the control systemmay update the quantity value of the item-size category descriptor forthe storage module from which the item was removed.

In some embodiments, the capacity of the storage module to holdadditional items in the stowability information may be updated 630 alsoby the control system, such as a control system implementing astowability information state manager 140 in FIG. 1. The update mayoccur in response to an item's removal from a storage module, applyingthe storage module capacity model to the updated item-size categorydescriptor. In other embodiments, the update may occur when a controlsystem accesses the stowability information.

Example Layout of a Materials Handling Facility with Mobile RoboticDevices

FIG. 7 illustrates an example of a physical layout of the stow area of amaterials handling facility in which mobile robotic devices maytransport storage modules to stow stations, according to someembodiments. The number, relative size, and arrangement of elements inFIG. 7 are given by way of example, and are not intended to be limiting.Materials handling facility 700 may include one or more inventory 710areas. Each inventory 710 area includes a stock storage 714 thatincludes a plurality of storage modules 720. Each storage module 720 mayinclude one or more locations (e.g., slots, shelves, partitions, bins,etc.) in which units of items are stowed. Storage modules 720 may haveseveral storage module types. For example, a storage module type may bea bay on a four-side storage unit. Several storage modules may belocated on a storage unit or apparatus. Each storage module 720 may stowone or more different types of items.

Each inventory 710 area also includes one or more stow stations 712. Inthis example, inventory 710 area includes six stow stations 712. At anyone time, one or more bots 722 may be moving storage modules 720 fromstock storage 714 area to stow stations 712 to stow items in storagemodules, under direction of control system 790. In at least someembodiments, to move a storage module 720, a bot 722 may move under thestorage module 720, lift the storage module 720, and then move thestorage module 720 to a destination (e.g., a particular stow station712) under direction of control system 790. FIG. 7 shows severalexamples of bots with storage modules 724. For example, FIG. 7 shows botwith storage module 724A moving from stock storage 714 towards a stowstation 712, bot with storage module 724B at a stow station 712, and botwith storage module 724C moving from a stow station 712B to stockstorage 714. FIGS. 8A and 8B illustrate an example bot, FIGS. 9A and 9Billustrate an example storage module, and FIG. 10 illustrates an examplebot with storage module.

A stow area may also include one or more receiving 780 operations orareas for receiving shipments of stock from various vendors. Receivedstock may be placed into stock storage 714 in one or more inventory 750areas. The receiving 780 operation may also receive and processreturned, purchased, or rented items from customers. At least some ofthese items may be restocked into an inventory 750 area.

In at least some embodiments, rather than stowing directly to stockstorage received inventory may be delivered to one or more stowingstations 712. Under direction of the control system 790, storage modules720 may be automatically moved from stock storage 714 to particularstowing stations, for example using the bots 722. At the stowingstation(s), the received inventory may be manually or automaticallyplaced in locations on the storage modules 720 under direction of thecontrol system 790. The storage modules 714 are returned from thestowing station(s) to stock storage 714, for example using the bots 722.In some embodiments, induction and stowing may be combined at one ormore of the stow stations 712 so that individual items for orders can beinducted into a conveyance mechanism (not pictured) from storage modules720 for delivery to one or more downstream processing stations (e.g.,sorting stations) and received items can be stowed to the same storagemodules 720 prior to the storage modules 720 being returned to stockstorage 714. Storage modules 714 may also be moved from stock storage714 to the induction/stowing station(s) specifically for stowing.

The various areas, operations and stations of materials handlingfacility 700 may be located in one building or facility, oralternatively may be spread or subdivided across two or more buildingsor facilities. In addition, materials handling facility 700 may includeone or multiple levels or floors. For example, materials handlingfacility 700 may include one, two, or more levels; in multi-level stowareas 700, a separate inventory 710 area may located on each of two ormore levels, and various downstream processing stations may be locatedon one or more of the levels.

The configuration of materials handling facility 700, including theconfiguration of inventory 710 area, as shown in FIG. 7 is given by wayof example, and is not intended to be limiting. Other configurations formaterials handling facility 700, and inventory 710 area are possible andcontemplated.

In addition, a materials handling facility may include two or moreinventory 710 areas, which may be similarly or differently configured. Amaterials handling facility may, for example, have two or more levels,with a separate inventory 710 area located on at least two differentlevels. In these configurations, bots 722 may be directed, eitherautomatically by control system 790 or by operator input to the controlsystem 790, from one inventory 710 area to another inventory 710 area.For example when a first inventory 710 area has high demand and needsmore bots 722, while a second inventory 710 area has less demand and canspare at least one bot 722, at least one bot 722 may be directed fromthe second inventory 710 area to the first inventory 710 area. Inmulti-level facilities, lifts may be provided to convey bots 722 betweenlevels as necessary or desired. A bot 722 may be remotely directed, bythe control system 790, to go to a lift, get on the lift, get off thelift at the appropriate level, and proceed to an inventory 710 area onthe level. In at least some embodiments, as an alternative to directingthe movement of bots 722 between inventory 710 areas to meet demandneeds, control system 790 may instead direct stows from one inventory710 area to another inventory 710 area or areas. For example, if thecontrol system 790 detects that a particular inventory 710 area isfalling behind, the control system 790 may shift some of the stow loadto another inventory area 710.

In some implementations, an inventory 710 area may be subdivided intozones, for example a zone of the stock storage area corresponding to(and nearby) each stow station, and one or more bots may be assigned tooperate within each zone. In these implementations, for example tohandle differences in demand, bots 722 may be directed, eitherautomatically by control system 790 or by operator input to the controlsystem 790, from one zone to another zone in the inventory 710 area, oralternatively stow load may be directed from one zone to another zone inan inventory 710 area.

FIGS. 8A and 8B illustrate an example mobile robotic device (bot) thatmay be used in at least some embodiments. The shape, size, andconfiguration of the bot 722 and its components are examples forillustrative purposes, and are not intended to be limiting. FIG. 8Ashows a top view of the example bot 722, and FIG. 8B shows a side viewof the bot 722. A bot 722 may include at least a body or chassis, a liftmechanism located on top of the body for lifting storage modules 720,and wheels (or other mechanisms such as tracks or treads) locatedunderneath the body. While FIG. 8A shows a circular lift mechanism, thelift mechanism may be other shapes. While FIGS. 8A and 8B show a bot 722with four wheels, a bot 722 may include more or fewer wheels, forexample two rows of three wheels, three rows with four wheels in eachrow, and so on. Also, different wheels may have different functions; forexample, one or more wheels may be drive wheels, one or more wheels maybe steerable wheels, and one or more wheels may just be free-rollingwheels. In some implementations, a bot 722 may be configured to raiseand lower the body relative to the wheels. In some implementations, abot may not include a separate lift mechanism as shown in FIGS. 8A and8B for lifting storage modules 720; instead, raising and lowering thebody relative to the wheels may act to lift and lower storage modules720.

A bot 722 may be short enough, with the lift mechanism lowered, to fitunder storage modules 720. In some embodiments, the bot 722 may be shortenough to pass under other components of the materials handlingfacility. As shown in FIG. 8B, a bot 722 may include at least one drivemotor for driving the wheels, at least one lift motor for raising andlowering the lift mechanism, communications and control components thatmay communicate (e.g., via wireless communications) with a controlsystem 790 or other external devices and control motions and actions ofthe bot 722 under direction of the control system 790 and/orautonomously, and one or more sensors that may, for example sense thecurrent location of the bot 722 in the materials handling facilityand/or the location of the bot 722 relative to other external objects,devices, or stations in the materials handling facility. Communicationsand control components may include at least one computing device similarin architecture to the example computer system 900 illustrated in FIG.12, which may store computer code and data for controlling the bot 722.The sensors may include optical sensors, tactile sensors, and/or othertypes of sensors. A bot 722 may also include at least one power supplyand/or connection for connecting to an external power source. In someembodiments, the power supply may be or may include a rechargeablebattery. A bot 722 may include other components, for example videocameras, audible alarms, external lights, access panels, externalcontrols such as on/off buttons, antennas, and Radio FrequencyIdentifier (RFID) devices. For example, a bot 722 may include an RFIDtag that uniquely identifies the bot 722 in the materials handlingfacility. However, other techniques or devices may be used to uniquelyidentify bots 722 in the materials handling facility.

A materials handling facility may include sensors, RFID readers, orother devices that enable the control system 790 to determine, and trackthe location of specific bots 722 in the facility via wired or wirelesscommunications. A materials handling facility may also includetransmitters or similar devices that enable the control system 290 todirect the motions and actions of specific bots 722. In someimplementations, a materials handling facility may include guide strips,tracks, or other techniques for guiding the movements of the bots 722 inthe facility, either autonomously or under control of the control system790. For example, the aisles in the inventory 710 area illustrated inFIG. 7 may have guide strips embedded in or attached to the surface ofthe floor for guiding the bots 722.

In some implementations, a materials handling facility may use two ormore different types or configurations of bots 722 for moving differenttypes or configurations of storage modules.

FIGS. 9A and 9B illustrate an example storage module that may be used inat least some embodiments. FIG. 9A shows a front view of an examplestorage module 720, and FIG. 9B shows a side view of the example storagemodule 720 of FIG. 9A. A storage module 720 may include one or morelocations 726 into which units of items may be stowed. In someimplementations, only one type of item is stowed in each location 726.However, implementations are possible in which two or more differenttypes of items may be stowed in a location 726. FIG. 9A shows an exampleconfiguration for a storage module 720 that may be used in someimplementations, and is not intended to be limiting. Storage modules 720may have different configurations and storage module types, and storagemodules 720 with different configurations may be used together in animplementation of an inventory 710 area. In various implementations,storage modules 720 may have different numbers of locations, and/ordifferent sizes of locations. In some implementations, a storage module720 may have two or more different sizes of locations to accommodatemore or fewer items and/or larger or smaller items. In some embodimentstwo or more storage modules may be combined into a storage unit. Whenstorage module is selected for a stow operation, the entire storage unitmay be transported.

As shown in FIGS. 9A and 9B, a storage module 720 may include legs thatraise the bottom shelf of the unit 720 high enough so that a bot 722 canfit under the unit 720. This example shows four legs arranged at thecorners of the unit 720; however, other numbers or configurations oflegs are possible.

Each storage module 720 may include at least one indicator 728associated with each location 726 of the storage module 720. Anindicator 728 may, for example, be used to indicate, at a stow station712, that a unit of an item is to be stowed into 726 under control ofcontrol system 790. The indicator 728 may be a light, an audio signal,or some other mechanism or combination of mechanisms capable ofconveying audible, visual, and/or other information to a human operator.As an alternative, or in addition, to indicators 728 physically locatedproximate to locations of storage module 720, textual and/or graphicaldirections may be displayed on a monitor of a computer, hand-helddevice, etc., or printed to paper output for operator(s) at a stowstation 712.

Each storage module 720 may include at least one identifier 729, whichmay be attached to or integrated in a leg (as shown) or attached to orintegrated in some other portion or surface of the unit 720. Theidentifier 729 may uniquely identify the storage module 720 in aninventory 710 area, or in the materials handling facility. For example,a storage module 720 may include an RFID tag that uniquely identifiesthe storage module 720 in the materials handling facility. However,other techniques or devices may be used to uniquely identify storagemodules 720.

FIG. 10 illustrates an example bot with a storage module, according toat least some embodiments. A bot 722, under control of control system790, may move underneath a storage module 790, and may raise the liftmechanism to lift the storage module 720 off the floor. The bot 722 maythen, under control of the control system and/or autonomously, move thestorage module 720 to another location, for example to a specific stowstation 712 of an inventory 710 area. A bot 722 with a storage modulelifted, as illustrated in FIG. 10, may be referred to as a bot withstorage module 724.

Example Layout of Materials Handling Facility for Transporting Items toInventory Area

FIG. 8 illustrates an example of a physical layout of the stow area of amaterials handling facility in which items may be transported to aninventory storage area, according to some embodiments. Materialshandling facility 800 is implementing a stow process in a materialshandling facility, such as described above with regard to FIG. 2 above.Items may be received at a materials handling facility 800 in receiving880. Some items may be identified as ready to be stowed 812. Controlsystem 890 may be a control system, such as described above with regardto FIG. 1. Inventory area 810 may include one or more storage modules820, such as the storage modules described above with regard to FIGS. 7through 10. Control system 890 may direct a stower 822 to transport areceived item 812 to a selected storage module and perform a stowoperation stowing the item into the selected storage module, accordingto the methods outlined in FIGS. 3 through 6 above. Control system 890may communicate stow directions to stowers 822 using various methods,including but not limited to paper print out, electronic display, aportable inventory reporting device, such as described above with regardto FIG. 1. Communications may travel on a wired or wireless network.

A stower 822 may be a human or robotic operator capable of stowing oneor more items into a storage module 820. Control system may 890 directstowers 822 to multiple selected storage modules 820. In someembodiments, stowers 822 may indicate to the control system 890 whenitems are stowed in a storage module 820, such that control system 890may update the stowability information for the storage module 820. Acontrol system 890 may direct stow operations for multiple inventoryareas 810 and balance the stow load of received items by selectingstorage modules 820 in different inventory areas 810.

The various areas, operations of stow area 800 may be located in onebuilding or facility, or alternatively may be spread or subdividedacross two or more buildings or facilities. In addition, stow area 800may include one or multiple levels or floors. For example, a stow area800 may include one, two, or more levels; in multi-level stow areas 800,a separate inventory 810 area may located on each of two or more levels,and various downstream processing stations may be located on one or moreof the levels.

The configuration of materials handling facility 800, including theconfiguration of inventory 810 area, as shown in FIG. 8 is given by wayof example, and is not intended to be limiting. Other configurations formaterials handling facility 800, and inventory area 810 are possible andcontemplated.

Example System

In at least some embodiments, a system that implements a control systemthat implements determining stowability based on item-size categoriesmay include a general-purpose computer system that includes or isconfigured to access one or more computer-accessible media, such ascomputer system 900 illustrated in FIG. 12. In the illustratedembodiment, computer system 900 includes one or more processors 910coupled to a system memory 920 via an input/output (I/O) interface 930.Computer system 900 further includes a network interface 940 coupled toI/O interface 930

In various embodiments, computer system 900 may be a uniprocessor systemincluding one processor 910, or a multiprocessor system includingseveral processors 910 (e.g., two, four, eight, or another suitablenumber). Processors 910 may be any suitable processors capable ofexecuting instructions. For example, in various embodiments, processors910 may be general-purpose or embedded processors implementing any of avariety of instruction set architectures (ISAs), such as the x86,PowerPC, SPARC, or MIPS ISAs, or any other suitable ISA. Inmultiprocessor systems, each of processors 910 may commonly, but notnecessarily, implement the same ISA.

System memory 920 may be configured to store instructions and dataaccessible by processor(s) 910. In various embodiments, system memory920 may be implemented using any suitable memory technology, such asstatic random access memory (SRAM), synchronous dynamic RAM (SDRAM),nonvolatile/Flash-type memory, or any other type of memory. In theillustrated embodiment, program instructions and data implementingdesired functions, such as those methods and techniques described abovefor a control system in a robotic induction technique and/or in anon-linear, unit-level materials handling system that implements therobotic induction technique, are shown stored within system memory 920as code 925 and data 926.

In at least some embodiments, I/O interface 930 may be configured tocoordinate I/O traffic between processor 910, system memory 920, and anyperipheral devices in the device, including network interface 940 orother peripheral interfaces. In some embodiments, I/O interface 930 mayperform any necessary protocol, timing or other data transformations toconvert data signals from one component (e.g., system memory 920) into aformat suitable for use by another component (e.g., processor 910). Insome embodiments, I/O interface 930 may include support for devicesattached through various types of peripheral buses, such as a variant ofthe Peripheral Component Interconnect (PCI) bus standard or theUniversal Serial Bus (USB) standard, for example. In some embodiments,the function of I/O interface 930 may be split into two or more separatecomponents, such as a north bridge and a south bridge, for example.Also, in some embodiments some or all of the functionality of I/Ointerface 930, such as an interface to system memory 920, may beincorporated directly into processor 910.

Network interface 940 may be configured to allow data to be exchangedbetween computer system 900 and other devices attached to a network,such as other computer systems, communications devices, controlmechanisms, readers, scanners and so on that are components of a controlsystem in a materials handling system that implements the roboticinduction technique. The communications channels may include, but arenot limited to conventional and mobile telephone and text messagingcommunications channels. Network interface 940 may commonly support oneor more wireless networking protocols (e.g., Wi-Fi/IEEE 802.11, oranother wireless networking standard). However, in various embodiments,network interface 940 may support communication via any suitable wiredor wireless general data networks, such as other types of Ethernetnetwork, for example. Additionally, network interface 940 may supportcommunication via telecommunications/telephony networks such as analogvoice networks or digital fiber communications networks, via storagearea networks such as Fibre Channel SANs, or via any other suitable typeof network and/or protocol.

In some embodiments, system memory 920 may be one embodiment of acomputer-accessible medium configured to store program instructions anddata as described above for FIGS. 1 through 11 for implementing acontrol system for, or possibly other components of, control system thatimplements determining stowability based on item-size categories.However, in other embodiments, program instructions and/or data may bereceived, sent or stored upon different types of computer-accessiblemedia. Generally speaking, a computer-accessible medium may includestorage media or memory media such as magnetic or optical media, e.g.,disk or DVD/CD coupled to computer system 900 via I/O interface 930. Acomputer-accessible medium may also include any volatile or non-volatilemedia such as RAM (e.g. SDRAM, DDR SDRAM, RDRAM, SRAM, etc.), ROM, etc,that may be included in some embodiments of computer system 900 assystem memory 920 or another type of memory. Further, acomputer-accessible medium may include transmission media or signalssuch as electrical, electromagnetic, or digital signals, conveyed via acommunication medium such as a network and/or a wireless link, such asmay be implemented via network interface 940.

Various embodiments may further include receiving, sending or storinginstructions and/or data implemented in accordance with the foregoingdescription upon a computer-accessible medium. Generally speaking, acomputer-accessible medium may include storage media or memory mediasuch as magnetic or optical media, e.g., disk or DVD/CD-ROM, volatile ornon-volatile media such as RAM (e.g. SDRAM, DDR, RDRAM, SRAM, etc.),ROM, etc., as well as transmission media or signals such as electrical,electromagnetic, or digital signals, conveyed via a communication mediumsuch as network and/or a wireless link.

The various methods as illustrated in the Figures and described hereinrepresent exemplary embodiments of methods. The methods may beimplemented in software, hardware, or a combination thereof. The orderof method may be changed, and various elements may be added, reordered,combined, omitted, modified, etc.

Various modifications and changes may be made as would be obvious to aperson skilled in the art having the benefit of this disclosure. It isintended that the invention embrace all such modifications and changesand, accordingly, the above description to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. A method, comprising: performing, by one or morecomputing devices: identifying an item-size category for each of aplurality of items in a materials handling facility, wherein saiditem-size category for each of the plurality of items is determined fromamong a plurality of different item-size categories according to sizeinformation for the item; determining an item-size category descriptorfor each of a plurality of storage modules, wherein the plurality ofstorage modules are fully-stowed storage modules, wherein the item-sizecategory descriptor includes a quantity value for each differentitem-size category, and each quantity value indicates a total quantityof items stored in the fully-stowed storage module categorized in arespective one of the different item-size categories; and generating astorage module capacity model that characterizes item-size categorydescriptors for fully-stowed storage modules.
 2. The method of claim 1,further comprising, for each of multiple storage modules at a materialshandling facility, tracking a capacity for the storage module to holdadditional items according to the generated storage module capacitymodel, wherein said tracking comprises updating the capacity in responseto an item being picked from the storage module.
 3. The method of claim2, further comprising selecting a storage module for a stow operationfrom among the multiple storage modules according to the capacities ofthe plurality of storage modules to hold additional items.
 4. The methodof claim 3, further comprising directing performance of the stowoperation for stowing items received at the materials handling facilityinto the selected storage module.
 5. The method of claim 4, furthercomprising determining an updated capacity for the selected storagemodule upon completion of the stow operation.
 6. The method of claim 1,wherein said generating the storage module capacity model comprises:mapping each item-size category descriptor as data points in ann-dimensional space, wherein each dimension of the n-dimensional spacecorresponds to a respective one of the different item-size categories,and each quantity value of the item-size category descriptor maps to adifferent dimension on the n-dimensional space; fitting a function tothe data points in the n-dimensional space to describe the determineditem-size category descriptors for the plurality of storage modules. 7.The method of claim 6, further comprising determining a capacity for astorage module to hold additional items according to the generatedstorage module capacity model, comprising: calculating a distance froman item-size descriptor of the storage module to the function for thestorage module capacity model, wherein the distance provides a measureof the capacity of the storage module to hold additional items.
 8. Themethod of claim 1, wherein the plurality of storage modules comprises aplurality of different storage module types, the method furthercomprising generating a different storage module capacity model for eachdifferent storage module type.
 9. A system, comprising: a plurality ofstorage modules at a material handling facility; and a control system,implemented by one or more computers, configured to: identify anitem-size category for each of a plurality of items in the materialshandling facility, wherein said item-size category for each of theplurality of items is determined from among a plurality of differentitem-size categories according to size information for the item;determine an item-size category descriptor for each of the plurality ofstorage modules, wherein the plurality of storage modules arefully-stowed storage modules, wherein the item-size category descriptorincludes a quantity value for each different item-size category, andeach quantity value indicates a total quantity of items stored in thefully-stowed storage module categorized in a respective one of thedifferent item-size categories; and generate a storage module capacitymodel that characterizes item-size category descriptors for fully-stowedstorage modules.
 10. The system of claim 9, wherein the control systemis further configured to: for each of the multiple storage modules atthe materials handling facility, track a capacity for the storage moduleto hold additional items according to the generated storage modulecapacity model, wherein said track a capacity comprises updating thecapacity in response to an item being picked from the storage module.11. The system of claim 10, wherein the control system is furtherconfigured to: select a storage module for a stow operation from amongthe multiple storage modules according to the capacities of theplurality of storage modules to hold additional items.
 12. The system ofclaim 11, wherein the control system is further configured to: directperformance of the stow operation for stowing items received at thematerials handling facility into the selected storage module; anddetermine an updated capacity for the selected storage module uponcompletion of the stow operation.
 13. The system of claim 9, wherein togenerate the storage module capacity module, the control system isfurther configured to: map each item-size category descriptor as datapoints in an n-dimensional space, wherein each dimension of then-dimensional space corresponds to a respective one of the differentitem-size categories, and each quantity value of the item-size categorydescriptor maps to a different dimension on the n-dimensional space; andfit a function to the data points in the n-dimensional space to describethe determined item-size category descriptors for the plurality ofstorage modules.
 14. The system of claim 13, wherein the control systemis further configured to determine a capacity for a storage module ofthe plurality of storage modules to hold additional items according tothe generated storage module capacity module, wherein to determine acapacity for a storage module to hold additional items according to thegenerated storage module capacity module, the control system is furtherconfigured to: calculate a distance from an item-size descriptor of thestorage module to the function for the storage module capacity model,wherein the distance provides a measure of the capacity of the storagemodule to hold additional items.
 15. A non-transitory, computer-readablestorage medium storing program instructions computer-executable toimplement a control system configured to: identify an item-size categoryfor each of a plurality of items in a materials handling facility,wherein said item-size category for each of the plurality of items isdetermined from among a plurality of different item-size categoriesaccording to size information for the item; determine an item-sizecategory descriptor for each of a plurality of storage modules, whereinthe plurality of storage modules are fully-stowed storage modules,wherein the item-size category descriptor includes a quantity value foreach different item-size category, and each quantity value indicates atotal quantity of items stored in the fully-stowed storage modulecategorized in a respective one of the different item-size categories;and generate a storage module capacity model that characterizesitem-size category descriptors for fully-stowed storage modules.
 16. Thenon-transitory, computer-readable storage medium of claim 15, whereinthe control system is further configured to: for each of the multiplestorage modules at the materials handling facility, track a capacity forthe storage module to hold additional items according to the generatedstorage module capacity model, wherein said track a capacity comprisesupdating the capacity in response to an item being picked from thestorage module.
 17. The non-transitory, computer-readable storage mediumof claim 16, wherein the control system is further configured to: selecta storage module for a stow operation from among the multiple storagemodules according to the capacities of the plurality of storage modulesto hold additional items.
 18. The non-transitory, computer-readablestorage medium of claim 17, wherein the control system is furtherconfigured to: direct performance of the stow operation for stowingitems received at the materials handling facility into the selectedstorage module; and determine an updated capacity for the selectedstorage module upon completion of the stow operation.
 19. Thenon-transitory, computer-readable storage medium of claim 15, wherein togenerate the storage module capacity model, the control system isfurther configured to: map each item-size category descriptor as datapoints in an n-dimensional space, wherein each dimension of then-dimensional space corresponds to a respective one of the differentitem-size categories, and each quantity value of the item-size categorydescriptor maps to a different dimension on the n-dimensional space; andfit a function to the data points in the n-dimensional space to describethe determined item-size category descriptors for the plurality ofstorage modules.
 20. The non-transitory, computer-readable storagemedium of claim 19, wherein the control system is further configured todetermine a capacity for a storage module of the plurality of storagemodules to hold additional items according to the generated storagemodule capacity module, wherein to determine a capacity for a storagemodule to hold additional items according to the generated storagemodule capacity module, the control system is further configured to:calculate a distance from an item-size descriptor of the storage moduleto the function for the storage module capacity model, wherein thedistance provides a measure of the capacity of the storage module tohold additional items.